In recent years, as an auxiliary power source for a commercial power grid (hereinafter referred to as “a grid” accordingly), an introduction of a distributed power source system (such as a fuel cell system or a photovoltaic cell system) capable of supplying powers to loads by an interconnection operation in parallel with a grid is advanced in power consumers. A distributed power source system includes distributed power sources and power conditioners.
Further, during a power failure of a grid, it is known a distributed power source system is capable of supplying powers to loads by a self-sustained operation offline to the grid.
On the other hand, it is necessary a distributed power source system capable of an interconnection operation prevents a state that an output power of the distributed power source system enables to make a reverse power flow to a grid (namely, an islanding operation) during a power failure of the grid.
Therefore, it is known a distributed power source system has an islanding operation prevention function detecting an abnormality of a power supplied to loads (for example, a power failure of a grid or an unstable state of a grid power) and stopping the power output (for example, refer to Patent Literature 1).